1. Field of the Invention
The present invention relates to an image-forming apparatus by so-called electrophotographic process, equipped with a toner container containing a set of toner particles and an endless belt circulating via certain transfer positions in contact with image carriers which carry electrostatic latent images, and to an image-forming method used in the image-forming apparatus.
2. Description of the Related Art
In image-forming apparatuses by the electrophotographic process such as copying machines and printers, an image carrier is first charged by an electrostatically charging device, before images are developed on the image carrier surface. Subsequently, an electrostatic latent image is formed on the surface of the image carrier, by exposing the image carrier to patterned light. Then, a toner image is formed on the image carrier by supplying the toner particles contained in the toner container onto the image carrier and developing the electrostatic latent image on the image carrier. The image is finally formed on a certain recording medium, by retransferring the toner image formed on the image carrier further onto the recording medium and fixing the image thereon. Some of these image-forming apparatuses employ an intermediate transfer belt method, wherein during transfer of the toner image formed on the image carrier onto a certain recording medium, the toner image is first transferred onto the surface of an endless belt circulating in a certain direction via the nip portions in contact with image carriers, and then the toner image transferred onto the surface of the belt is retransferred onto the recording medium. The intermediate transfer belt method is often employed, when multiple images in color are transferred one by one superimposed at the respective nip portions onto the surface of a belt (hereinafter, referred to as intermediate transfer belt) and the resulting superimposed multi-color image is transferred onto a recording medium.
Hitherto, elastic belts consisting of an woven fabric made of polyester or the like and an elastic layer formed thereon (see e.g., Japanese Patent Application Laid-Open (JP-A) No. 9-305038 and JP-A No. 10-240020, and others), and polyimide belts made of a polyimide resin superior in mechanical property and heat resistance (see e.g., JP-A No. 10-63115 and others) have been proposed as the intermediate transfer belts.
Recently, there exists a need for higher-image quality similar to photographic image quality in the copying machines and printers employing the electrophotographic process, while in image-forming apparatuses employing the intermediate transfer belt method, there is a need for prevention of out-of-color registration during the transfer of multiple images different in color one by one superimposed onto the intermediate transfer belt, and of scattering of toner particles (blur) during the transfer of the images onto the intermediate transfer belt.
In the image-forming apparatuses employing as the intermediate transfer belt an elastic belt described in Japanese Patent Application Laid-Open (JP-A) No. 9-305038 and JP-A No. 10-240020, the elastic layer of the intermediate transfer belt deforms along the surface of the image carrier and covers the toner particles on the image carrier, thus preventing the blur of toner particles, but due to an insufficient mechanical strength of the intermediate transfer belt, the belt is easily deformed by the stress applied during the belt movement, causing the out-of-color registration in the transfer region, i.e., at the nip portion. Thus, such image-forming apparatuses cannot provide the high-quality image demanded in recent years. Alternatively, in the image-forming apparatus employing an conductive polyimide belt described in JP-A NO. 10-63115 as the intermediate transfer belt, because of its higher mechanical strength of the intermediate transfer belt, the belt does not deform by the stress of movement, and consistently provides high-quality images without out-of-color registration, but the surface of the belt is too hard that the intermediate transfer belt cannot deform along the surface of the image carrier or cover the toner particles on image carrier with the belt at the nip portion, thus causing more frequent occurrence of the blur of toner particles. The frequency of the blur of toner particles depends also on the diameter of toner particles, and thus it is necessary to consider the properties of the intermediate transfer belt as well as the properties of toner particles, for obtaining high-quality images.